The purpose of this project is to provide new knowledge of the auditory mechanisms of the inner ear. We have continued our studies on the immunocytochemical localization of neurotransmitter candidates and enzymes in the auditory sensory organ. Small mammals (guinea pigs, rats and mice) served as research subjects. Antisera to choline acetyltransferase, aspartate aminotransferase, glutaminase and enkephalins have given positive antigen-antibody reactions. Negative findings have resulted from the use of antisera to substance P, ACTH, LHRH, alpha MSH, neurotensin and somatostatin. The results have given no reliable clue regarding the nature of the neurotransmitter(s) that the hair cells of the auditory sensory organ may use. On the other hand, the case for the cholinergic nature of efferent, olivocochlear fibers has been strengthened: we have found choline acetyltransferase-like immunoreactivity in synaptic regions under both inner and outer hair cells. Also, ongoing biochemical studies indicate there are at least three opioid peptides in the auditory sensory organ of the guinea pig. We now have also demonstrated co-containment of enkephalin-like immunoreactivity and acetylcholinesterase in brainstem cells of origin of a subpopulation of the cochlear efferents of the guinea pig, using immunocyto-chemical and histochemical techniques and light microscopy. We believe with many that synaptic activity of efferents at outer hair cells changes the micromechanics of the auditory sensory organ. We will try to find out how, if at all, such activity may change the microstructure of the outer hair cell. We continue to look for markers of the different parts of the two types of spiral ganglion cells of the auditory nerve. For this reason, and others, we have studied the postnatal development of the spiral ganglion cells of the rat. A study is in progress on the development of glutaminase-like and asparatate aminotransferase-like immunoreactivity of spiral ganglion cells.